1. Field of the Invention
The invention relates generally to fluidized beds and, more particularly, to a fluidized bed with immersion heat exchangers immersed in the combustion bed thereof.
2. Description of the Prior Art
In fluidized beds, coal mixed with fine-grained sand, lime and ash particles is typically burned in a suspended state. The combustion bed is typically comprised of one or more fluidization regions or cells. The air for supporting combustion in a fluidized bed is typically introduced into the cell or cells through a nozzle or series of nozzles in the bottom of the bed. The combustion air rises through the combusting material and places the bed material in turbulent motion. Heat exchanging tubes or lines, which may produce steam or heat water, are typically immersed in the fluidized bed and transfer approximately up to fifty percent of the heat generated in the bed therefrom. By the action of this relatively high heat transfer from the bed to the heat exchanging lines, the bed temperatures are maintained at a relatively low level. Because of the turbulent motion of the combusting materials within the bed, the transfer of heat of combustion to these tubes is very great. Typically, disposed above the fluidized bed, there is a space which may be open and which is used for after-burning of the products that emanate from the fluidized bed. The exhaust gases from this open space are typically conducted into a convection heat transfer arrangement, such as in a boiler or steam generator of a conventional design well known in the art.
The combustion temperatures in fluidized beds are typically between about 800.degree. C. to about 900.degree. C., and preferably between 800.degree. C. and 900.degree. C. In this temperature range, the sulfur contained in the coal combines with limestone present in the bed. The result of this combination of limestone and sulfur is a dry, inert waste product which is primarily gypsum, which can be deposited with, and removed with, the ash produced in the bed. Typically, 80 to 90 percent of the sulfur in the coal is bound with the limestone and removed through the ash. Significantly, because of the low combustion temperatures, the NO.sub.x emissions are also significantly reduced over other means of combustion at higher temperatures. Therefore, pollution generated in fluidized bed combustion is usually significantly lower in gaseous contaminants than combustion in other types of installations. Dusts generated by the fluidized bed process are retained in a cyclone separator, typically, which is usually followed by a cloth filter. Another advantage of fluidized bed combustion is that all sorts and grades of coal, even those with a high ash content, can be burned therein without any appreciable problems.
However, a disadvantage of fluidized bed combustion resides in the relatively low output of heat in thermal units per unit of volume of the bed. This limitation of current designs is especially true for a unit which operates under atmospheric pressure conditions. Current efforts are being made to develop units having efficiencies higher than those typically presently existing in atmospheric pressure beds by applying pressurized operation to fluidized beds in so-called pressurized fluidized bed installations. Another means of increasing the efficiency is by the use of circulating fluidized beds. The pressurized fluidized bed, in contrast to the atmospheric fluidized bed, is operated at a significant pressure, greater than atmospheric pressure. In the pressurized fluidized bed, the problem of bubble formation, also encountered in atmospheric fluidized beds, is even more pronounced. These bubbles rise through the fluidized bed and interfere very substantially with the operation of the fluidized bed. These ascending bubbles, among other phenomena, produce an undesirable acceleration on the solid particles within the bed and an undesirable velocity of these particles when they leave the fluidized bed, such that the particles move beyond the after-burning zone and into the subsequent filters. This discharge of solid particles from the fluidized bed is very undesirable. In a circulating fluidized bed, the discharge of particles is promoted by the action thereby. By the filtering action, however, the recycling of unburned solid particles which have been discharged by the fluidized bed is substantially assured. Stated otherwise, the solid particles are transported in a turbulent movement in the circuit including the fluidized bed.
An example of the prior art regarding atmospheric fluidized beds is disclosed in U.S. Pat. No. 4,425,302, entitled "Fluidized Bed Reactor for Particulate Material". This U.S. patent claims priority from German Patent Publication Published for Opposition Purposes No. DE-OS 31 01 942. In this U.S. patent, the above-mentioned gas bubbles can be at least partially prevented by inserts in the form of louvers. These louvers form baffles which are comprised of sheets which stand either horizontally or are inclined downwardly to the lowest point of the combustion wall. With these louver-like baffles, the flow in the fluidized bed is deflected. Unfortunately, these baffles do not prevent the erosion of the metal outer heat exchanging immersion surfaces of the heat exchanging tubes or lines. Erosion also occurs in other types of fluidized bed installations. This erosion results from the friction of the solid particles against the preferably metal heater surfaces which are immersed in the combusting materials in the fluidized bed. Another prior art fluidized bed is disclosed in U.S. Pat. No. 4,545,959, entitled "Treatment Chamber with Fluidized Bed". Both the U.S. patents and the German Patent Publication Published for Opposition Purposes cited above are incorporated herein by reference as if set forth in full in the text of this application.